In a combustion engine associated with a vehicle or other equipment, if evaporated fuel in the fuel tank leaks into the atmosphere, it becomes one of the causes for air pollution because a large quantity of hydrocarbons (HC) is contained in the evaporated fuel, and it also results in fuel loss. As a means for solving the problems described above, an evaporated fuel controller is used wherein evaporated fuel is absorbed by and stored in a canister with an absorbent such as activated carbon packed therein. The evaporated fuel absorbed by and stored in the canister is then released for combustion in an engine.
An evaporated fuel controller as described above is disclosed in Japanese Patent Laid Open Publication No. 268861/1986. The controller disclosed in this publication is used in a multicylinder combustion engine in which each cylinder has a suction pipe with a throttle valve provided therein. The suction pipes are communicated to each other by a communicating tube downstream from each throttle valve. A plurality of inlet ports each having a different passage area are provided in the aforesaid communicating tube to switch and communicate a purge port to the plurality of inlet ports by a switching valve according to the load of the combustion engine.
A conventional type of evaporated fuel controller is shown in FIG. 2 wherein 202 indicates a combustion engine, 204 indicates an air intake path, 206 indicates a throttle valve, 208 indicates an exhaust path, and 210 indicates a fuel tank. An evaporated fuel controller 212 in this combustion engine communicates the fuel tank 210 with a canister 214 by an intake path 216. The canister 214 is communicated to the air intake path 204 by a release path 218. The release path 218 comprises a main path section 220, a first branch path section 222, and a second branch path section 224. The main path section 220 extends from the canister 214 to a branch point 226. The first branch path section 222 extends from the branch point 226 to a first port 228 provided in the air intake path 204 downstream of the throttle valve 206. The second branch path section 224 extends from the branch point 226 to a second port 230 provided in the air intake path 204 downstream of the first port 228.
As described above, the release path 218 is communicated at one end thereof with the canister 214 and at the other end to the first port 228 and the second port 230.
In line with the first branch path section 222 is a high load control valve 232 which is opened or closed by a controlling section (not shown) so that evaporated fuel absorbed by and stored in the canister 214 is desorbed and released to the combustion engine for combustion under a high load. In line with the second branch path section 224 is a low load control valve 234 which is opened or closed by the controlling section mentioned above so that fuel absorbed by and stored in the canister 214 is desorbed and released to the combustion engine 202 for combustion under a low load.
In the evaporated fuel controller having the construction described above, however, the high load control valve 232 and the low load control valve 234 are opened or closed independently, and the evaporated fuel can not be released in an appropriate quantity in a range from a low load state to a high load state of the combustion engine 202, which may detrimentally cause fluctuations in the air/fuel ratio.
Also, sometimes a leakage diagnosis (i.e. a diagnosis on leakage of evaporated fuel from inside the device into the atmosphere) is required to be performed on an evaporated fuel controller. In the leakage diagnosis as described above, sometimes leakage of fuel from a component causes a problem. If a large quantity of evaporated fuel leaks from any component, it may cause an erroneous determination in the leakage diagnosis. In the evaporated fuel controller shown in FIG. 2, the high load control valve 232 and the low load control valve 234 are provided in parallel in the release path 218, so that a leakage rate specific to the control valve is doubled, and for this reason a quantity of evaporated fuel leaked from the high load control valve 232 and the low load control valve 234 each having the defect as described above doubles, which makes a precise determination of leakage difficult.
It should be noted that in some evaporated fuel controllers only a high load control valve is provided in the release path. In this type of evaporated fuel controller, inasmuch as a low load control valve is not provided therein, a precise flow rate of evaporated fuel released in a low load state such as in idling can not be insured, so that release of evaporated fuel is not carried out in a low load state in this type of evaporated fuel controller.
In order to solve the problems as described above, the evaporated fuel controller according to the present invention is characterized by a release path, one end of which is communicated to a canister for absorbing and storing evaporated fuel, and the other end of which branches into two paths at a branch point. A first branch path is communicated to an air intake path downstream from a throttle valve of a combustion engine, and a second branch path is communicated to the air intake path upstream from the aforesaid throttle valve. A high load control valve which is opened or closed in a range from a low load state to a high load state of the aforesaid combustion engine is provided in line with the release path communicating the aforesaid canister to the branch point. A low load control valve which is opened or closed in a low load state of the aforesaid combustion engine in association with the aforesaid high load control valve is provided in line with the second branch path and communicates the aforesaid branch point to the air intake path upstream from the throttle valve.
In the configuration according to the present invention, the high load control valve and the low load control valve can correlatively be opened or closed in a range from a low load state to a high load state of the combustion engine.
Also, a diagnosis can be executed more precisely by taking into account leakage from only one of the load control valves (as compared to a conventional type of system in which a high load control valve and a low load control valve are provided in parallel).
Furthermore, evaporated fuel can be diluted by mixing air introduced by the low load control valve from an air intake path upstream from the throttle valve with evaporated fuel released in a low load state of a combustion engine by the high load control valve.